I want to do this (http://www.nethall.gr).. The homepage has a preview of the post and then when you get into the post there is the full text and the original size of the image… I know that it is easy but i can’t find the way to do this.. I know that the thumbnail can be inserted with custom fields but i try it so many times and i don’t have what i want…

You can hardcode/use post meta values instead of the featured image but the image wouldn’t be part of the post preview itself then (part of the text content). If you refer to a different featured image for previews you can implement this feature by editing index.php (and archive.php).